This invention relates to a new and practical process for preparing a 2-aralkyl-5,5-dialkyl-1,3,2-dioxaphosphorinan-2-one represented by the formula: ##STR1## in which R is an alkyl group having 1 to 2 carbon atoms and R' is an alkyl group having 1 to about 9 carbon atoms, and Ar is an aryl group having 6 to about 18 carbon atoms.
Compounds represented by formula (I) are useful flame retardants for certain plastics, as disclosed, for example, by A. Granzow in U.S. Pat. No. 4,288,063 of Oct. 14, 1980, incorporated herein by reference. A particularly useful property of these compounds is that they are efficient in imparting flame retardance at modest use levels while having a minimal adverse effect on the useful properties of the plastic, such as an undesired softening or plasticizing effect. Moreover, these compounds do not require the joint use of halogenated additives such as chlorinated paraffin or brominated phosphate esters, which aside from their cost, can have their own adverse effect on the properties of the plastic.
K. D. Bartle and coworkers in 1967 disclosed a preparation of the compound having formula I in which R and R' are methyl and Ar is phenyl (Tetrahedron, vol. 23, p. 1702) by heating 5,5-dimethyl-2-methoxy-1,3,2-dioxaphosphorinane (prepared by transesterification between trimethyl phosphite and 2,2-dimethylpropane-1,3-diol) with benzyl chloride for 12 hours at 170.degree.-180.degree. C. The yield is stated to be 60% of theoretical. As a practical method of preparation, however, the synthesis of Bartle is unsatisfactory, because the first intermediate, trimethyl phosphite, is difficult to prepare and highly volatile and odorous, and the yield in the transesterification with 2,2-dimethylpropane-1,3-diol is only 50%, making the overall yield (from trimethyl phosphite) only 30%.
Bartle's synthesis can be represented by the reaction equation: ##STR2## It can be seen that the second step of this sequence is an instance of the Arbuzov reaction in which the methyl group of the 5,5-dimethyl-2-methoxy-1,3,2-dioxaphosphorinane is displaced as methyl chloride. The art, however, has recognized that dioxaphosphorinanes with larger alkoxy substituents do not undergo displacement in the same way; instead, phosphonates are formed in which the dioxaphosphorinane ring is opened and no alkyl halide is produced. As a source of this statement, Bartle and coworkers cite Wadsworth and Emmons (J. Amer. Chem. Soc. 1962, vol. 84, page 610) and the latter authors in turn cite a 1958 paper by A. E. Arbuzov, the discoverer of the reaction named for him, in which displacement of an organic halide on a phosphite ester links the organic group of the halide to phosphorus through carbon while an organic group of the phosphite ester is displaced as halide.
Consequently, one is led to conclude that a 2-alkoxy-1,3,2-dioxaphosphorinane reactant with a benzyl halide for making a 2-benzyl 1,3,2-dioxaphosphorinan-2-one must be a 2-methoxy-1,3,2-dioxaphosphorinane, since 1,3,2-dioxaphosphorinanes with larger 2-alkoxy substituents would give a different and undesirable type of product.
A. Granzow in U.S. Pat. No. 4,288,063 of Oct. 14, 1980 also disclosed the preparation of compounds which can be represented by formula (I) in which R and R' are methyl and Ar is phenyl and 2,4,6-trimethylphenyl, by heating the aralkyl chloride with neopentyl phosphite and a strong base (sodium hydride) in dimethylformamide (DMF) solvent. The course of these preparations can be illustrated by the following reaction equations: ##STR3##
Granzow describes a 65% yield for the reaction with 2,4,6-trimethylbenzyl chloride and does not indicate the yield for the reaction with benzyl chloride. In any event, the need to use expensive sodium hydride and dimethylformamide and to handle and dispose of by-product hydrogen gas with its attendant safety hazards make Granzow's method impractical for industrial use.